The now well-known high intensity sodium vapor lamp is described in U.S. Pat. No. 3,248,590 -- Schmidt, 1966. "High Pressure Sodium Vapor Lamp,", and generally comprises an outer vitreous envelope or jacket of glass within which is mounted a slender tubular ceramic arc tube. The arc tube is made of a light transmissive refractory oxide material resistant to alkali metals at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to the electrodes. The outer envelope which encloses the ceramic arc tube is generally provided at one end with a screw base comprising shell and center contact to which the electrodes of the arc tube are connected.
The high pressure sodium vapor lamp contains an excess amount of sodium mercury amalgam, that is it contains more amalgam than is vaporized when the lamp reaches a stabilized operating condition. By having an excess, the vapor pressure is determined by the lowest operating temperature at any point in the arc tube and the quantity supplied is not critical. As the lamp ages, some of this excess amalgam is needed to replace that lost during the life of the lamp, for instance by electrolysis through the alumina walls.
In some lamps wherein the arc tube is symmetrical end for end, sometimes referred to as a universal burning design, the cold spot where the excess amalgam collects is located within the arc tube proper. An example of such a design is described in U.S. Pat. No. 3,609,437 -- Tol et al., wherein the arc tube has no exhaust tube and the amalgam charge is inserted into the arc tube just prior to sealing the second end closure within an inert gas-filled furnace. In such a design, the position of the excess amalgam when the lamp is operating is determined by temperature and gravity. The excess amalgam migrates to the coolest spot within the arc tube and gravity pulls it to the lowest position possible, generally to the closure at the lower end which is directly exposed to electrode heat. Deposition of electrode material on the arc tube walls during life tends to darken them and darkening is greatest at the ends near the electrodes. The resulting oven effect raises the temperature of the cold spot, causing more sodium to be vaporized which in turn causes lamp voltage to rise. It is a general characteristic of high pressure sodium lamps that the lamp operating voltage increases with life and the end of life occurs when the voltage supplied by the ballast is no longer sufficient to sustain lamp operation. At this point the lamp may cease to operate altogether or will cycle on and off due to the high voltage starting pulse supplied by the ballast. Thus the life of high pressure sodium lamps is dependent upon the rate of voltage rise. In prior art universal burning lamps, the oven effect aggravates voltage rise with the result that such lamps are relatively short lived.
In another well-known lamp design illustrated in U.S. Pat. No. 3,708,710 -- Smyser et al., the excess sodium mercury amalgam is condensed in a reservoir external to the arc tube proper. This contruction utilizes at least one tubular inlead of niobium which is used as an exhaust tube and has an opening into the interior of the arc tube. After the lamp has rceived its filling, the exhaust tube is hermetically tipped off and the heat balance is such that the tipped end becomes the cold spot in which the excess amalgam collects. The excess amalgam is now in the location removed from the direct heat of the arc and of the electrode, and arc tube blackening as the lamp ages now has a minimal effect on sodium vapor pressure and on lamp voltage. Also the use of an external reservoir facilitates fine tuning the heat balance, for instance by grit blasting the reservoir to regulate the heat loss in order to adjust the temperature to the optimum for lumen output and long life.
The external reservoir construction has had the drawback that the exhaust tube must be located lowermost. This has necessitated two versions of a given lamp, a base-up and a base-down design, the arc tube being inverted relative to the jacket in one as against the other. If either version is used in the incorrect orientation, vibration or mechanical shock may cause a droplet of amalgam to drop out of the exhaust tube into the arc tube. Since the arc region is at a much higher temperature, there will be a sudden rise in sodium and mercury vapor pressures and a corresponding increase in lamp voltage. This can be severe enough to cause the lamp to extinguish when the lamp voltage exceeds the maximum sustaining voltage of the ballast. There are many applications where such interruption of light or blinking cannot be tolerated. In extreme cases, the relatively cool amalgam droplet has been known to cause thermal cracking of the arc tube when its strikes, thereby ending the useful life of the lamp.